1. Field of the Invention
The present invention relates to a process for producing an oxide magnetic material having soft magnetism, particularly Mnxe2x80x94Zn ferrite. More particularly, the invention relates to a process for producing Mnxe2x80x94Zn ferrite which is suitable for use as low loss materials used in switching power supply transformer, flyback transformer or deflection yoke, various inductance elements, impedance elements for EMI countermeasure, electromagnetic wave absorbers, and the like.
2. Background of Related Art
There is Mnxe2x80x94Zn ferrite as the representative oxide magnetic material having soft magnetism. This Mnxe2x80x94Zn ferrite generally has a composition comprising basic components of more than 50 mol % of Fe2O3, 52 to 55 mol % of Fe2O3 on the average, 10 to 24 mol % of ZnO and the remainder being MnO. The Mnxe2x80x94Zn ferrite has conventionally been produced by mixing each raw material powder of Fe2O3, ZnO and MnO in predetermined proportion, forming the resulting mixture into a predetermined shape through each step of calcination, milling, component adjustment, granulation, pressing and the like, and then subjecting the green compact to sintering treatment such that the green compact is maintained at 1,200 to 1,400xc2x0 C. for 3 to 4 hours in a reduced atmosphere having greatly decreased oxygen concentration by flowing of nitrogen gas. The reason for sintering in a reduced atmosphere is that since the green compact contains Fe2O3 in a large amount of 50 mol % or more, if it is sintered in the air, densification does not proceed sufficiently, and as a result, good soft magnetism is not obtained. Further, Fe2+ to be formed by reduction of Fe3+ has a positive crystal magnetic anisotropy, and therefore has the effect that it offsets a negative crystal magnetic anisotropy of Fe3+, thereby increasing soft magnetism. However, if sintered in the air, formation of Fe2+ by such a reduction reaction cannot be expected.
However, it is known that the densification depends on oxygen concentration at the time of temperature rising in sintering, and formation of Fe2+ depends on oxygen concentration at the time of temperature dropping after sintering. Accordingly, if setting of oxygen concentration in sintering is mistaken, it is difficult to secure good soft magnetism. For this reason, conventionally the following equation (1) has been established experimentally, and oxygen concentration in sintering has been administered according to this equation (1).
log Po2=xe2x88x9214,540/(T+273)+bxe2x80x83xe2x80x83(1)
wherein T is temperature (xc2x0 C.), Po2 is oxygen concentration, and b is a constant. The term xe2x80x9coxygen concentrationxe2x80x9d in the present specification means the proportion of oxygen gas when the volume of all gases is set to 1, and has the same meaning as partial pressure of oxygen. The numerical value of about 7-8 has conventionally been employed as the constant b. The constant b being 7-8 means that oxygen concentration during sintering must be controlled to a narrow range, and due to this fact, there have conventionally been the problems that sintering treatment is very complicated and production cost is very high.
On the other hand, where Mnxe2x80x94Zn ferrite is used as a magnetic core material, eddy current flows with becoming a frequency used high, and loss by such an eddy current increases. Therefore, in order to raise the upper limit of the frequency that can be used as a magnetic core material, it is necessary to make its electrical resistance large as much as possible. However, there have been the problems that the electrical resistance in the above-described conventional Mnxe2x80x94Zn ferrite is a value smaller than 1 xcexa9m due to enjoyment of electron between Fe3+ and Fe2+ (interionic) as mentioned above, frequency that can be used is up to about several hundred kHz, and initial permeability remarkably decreases in a high frequency region exceeding 1 MHz, resulting in losing properties as soft magnetic material.
The present invention has been made in view of the above-described conventional problems. An object of the present invention is to provide a production process which can easily and inexpensively obtain Mnxe2x80x94Zn ferrite which has large electrical resistance and is sufficiently durable to the use in a high frequency region exceeding 1 MHz.
The above object can be achieved by the following aspects.
According to a first aspect of the present invention, there is provided a process for producing Mnxe2x80x94Zn ferrite, which comprises pressing a mixed powder comprising components adjusted so as to have a composition of 44.0 to 50.0 mol % of Fe2O3, 4.0 to 26.5 mol % of ZnO, 0.1 to 8.0 mol % of at least one member selected from the group consisting of TiO2 and SnO2, and the remainder being MnO, sintering the resulting green compact in the air, and then cooling the green compact in the air.
According to a second aspect of the present invention, there is provided a process for producing Mnxe2x80x94Zn ferrite, which comprises pressing a mixed powder comprising components adjusted so as to have a composition of 44.0 to 50.0 mol % of Fe2O3, 4.0 to 26.5 mol % of ZnO, 0.1 to 8.0 mol % of at least one member selected from the group consisting of TiO2 and SnO2, 0.1 to 16.0 mol % of CuO and the remainder being MnO, sintering the resulting green compact in the air, and then cooling the green compact in the air like the first aspect of the present invention.
According to a third aspect of the present invention, there is provided a process for producing Mnxe2x80x94Zn ferrite, which comprises pressing a mixed powder comprising components adjusted so as to have a composition of 44.0 to 50.0 mol % of Fe2O3, 4.0 to 26.5 mol % of ZnO, 0.1 to 8.0 mol % of at least one member selected from the group consisting of TiO2 and SnO2, and the remainder being MnO, sintering the resulting green compact in an atmosphere having an oxygen concentration as defined by the following equation, and then cooling the green compact after sintering the same at a temperature up to at least 300xc2x0 C.:
log Po2=xe2x88x9214,540/(T+273)+b
wherein T is temperature (xc2x0 C.), Po2 is oxygen concentration, and b is a constant selected from the range of 6 to 21. In this case, if the temperature is lower than 300xc2x0 C., since the reaction of oxidation and reduction can be disregarded without depending on the oxygen concentration, the adjustment of the atmosphere is sufficient such that the cooling after sintering advances to the point of 300xc2x0 C.
According to a fourth aspect of the present invention, there is provided a process for producing Mnxe2x80x94Zn ferrite, which comprises pressing a mixed powder comprising components adjusted so as to have a composition of 44.0 to 50.0 mol % of Fe2O3, 4.0 to 26.5 mol % of ZnO, 0.1 to 8.0 mol % of at least one member selected from the group consisting of TiO2 and SnO2, 0.1 to 16.0 mol % of CuO and the remainder being MnO, sintering the resulting green compact in an atmosphere having an oxygen concentration as defined by the following equation, and then cooling the green compact after sintering the same at a temperature up to at least 300xc2x0 C.:
log Po2=xe2x88x9214,540/(T+273)+b
wherein T is temperature (xc2x0 C.), Po2 is oxygen concentration, and b is a constant selected from the range of 6 to 21. In this case, if the temperature is lower than 300xc2x0 C., since the reaction of oxidation and reduction can be disregarded without depending on the oxygen concentration, the adjustment of the atmosphere is sufficient such that the cooling after sintering advances to the point of 300xc2x0 C.
It is known that iron component in Mnxe2x80x94Zn ferrite is present in the form of Fe3+ and Fe2+, but Ti and Sn form Fe2+ by receiving electron from Fe3+. Therefore, Fe2+ can be formed even by sintering in the air or an atmosphere containing an appropriate amount of oxygen by containing Ti and Sn.
The first to fourth aspects of the present invention make it possible to obtain good soft magnetism by that the content of TiO2 and/or SnO2 in the basic components is adjusted to 0.1 to 8.0 mol % to control the amount of Fe2+ to be formed and optimize a co-existence ratio of ratio of Fe3+ and Fe2+, thereby canceling out positive and negative crystal magnetic anisotropy. Further, according to the first to fourth aspects of the present invention, since Ti4+ and Sn4+ having the stable number of valency are present in large amount, exchange of electrons between Fe3+ and Fe2+ is substantially inhibited, and as a result, an electrical resistance that is considerably larger than the conventional electrical resistance can be obtained (about 103 times). However, if the content of TiO2 and/or SnO2 is less than 0.1 mol %, such an effect is small. On the other hand, if the content is larger than 8.0 mol %, the initial permeability decreases. For this reason, the content of TiO2 and/or SnO2 is adjusted to the range of 0.1 to 8.0 mol %.
In the first to fourth aspects of the present invention, since the content of Fe2O3 is suppressed to 50 mol % or less, even if sintered in the air or an atmosphere containing an appropriate amount of oxygen, densification proceeds sufficiently, so that the desired soft magnetism is obtained. However, if the content of Fe2O3 is too small, it results in decrease ininitial permeability. Therefore, Fe2O3 should contain in an amount of at least 44.0 mol %.
ZnO affects Curie temperature or saturation magnetization. If the content of ZnO is too large, Curie temperature lowers, resulting in practical problem. On the other hand, if the content of ZnO is too small, saturation magnetization decreases. For this reason, the content of ZnO is adjusted to the range of 4.0 to 26.5 mol %.
In the second and fourth aspects of the present invention, CuO is contained as described above. This CuO has the effect which enables low temperature sintering. However, if the content of CuO is too small, the above-described effect is small. On the other hand, if the content is too large, the initial permeability decreases. Therefore, the content of CuO is adjusted to the range of 0.1 to 16.0 mol %.
The first to fourth aspects of the present invention can contain a slight amount of at least one member selected from the group consisting of CaO, SiO2, ZrO2, Ta2O5, HfO2, Nb2O5 and Y2O3 as additives. Those additives have the function to make grain boundary highly resistant.
Further, at least one member selected from the group consisting of V2O5, Bi2O3, In2O3, PbO, MoO3 and WO3 can be contained as the additives. Those additives each are oxides having low melting point and have the function to accelerate sintering.
Additionally, at least one of Cr2O3 and Al2O3 may further be contained as the additive. Those additives have the function to improve temperature characteristics of initial permeability.
In the third and fourth aspects of the present invention, cooling after sintering is conducted in an atmosphere having an oxygen concentration determined using an optional value within the range of 6 to 21 as the constant b in the above-described equation (1). If a value larger than 21 is selected as the constant b in the equation (1), the atmosphere is substantially the same atmosphere as the air, and thus it is meaningless to define the oxygen concentration. Further, in order to increase the initial permeability at low frequency of Mnxe2x80x94Zn ferrite to be obtained, it is desirable to select a value as small as possible for the constant b. However, if the value is smaller than 6, electrical resistance becomes too small, and as a result, the initial permeability in high frequency region deteriorates. Therefore, the constant b is set to 6 or more.